Greece, Rome, and England were all by far the most influential civilizations in their golden eras but as it stands currently, the United States is by far the most influential nation and has done more to shape the world on a global scale than any other country in history.
Just think about it, what might the world look like today if the United States wasn't there to tip the scale in WWI, beat the tar out of the Axis in WWII (with the help of the other Allies of course), rebuild Japan and help it to become the technological juggernaut that it is today, help rebuild post WWII Europe, spearhead the founding of the UN and push for the creation of modern day Israel and Palestine, act as a counter balance to keep the Soviet union in check, help Afghanistan halt the Soviet invasion, help Iraq resist invasion by Iran, help defend South Korea from the North Korean and Chinese onslaught, building the most used canal in the world through Panama, inventing the computer, invading and liberating Afghanistan and Iraq, and those are just some highlights.
That's not too shabby for a country thats only been in existence for around 200 years.